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GHSA-GX3V-Q759-G323

Vulnerability from github – Published: 2026-07-21 21:57 – Updated: 2026-07-21 21:57

Summary

Gitea: TOTP TOCTOU race on web 2FA paths + missing replay check on Basic-Auth `X-Gitea-OTP` surface

Details

Summary

I'm reporting two related TOTP one-time-use defects in Gitea that survive the CVE-2021-45331 fix. The 2018 fix (PR #3878) introduced the TwoFactor.LastUsedPasscode field and added an in-memory inequality check on the web 2FA login path. That check works correctly in the single-request case, but it leaves two follow-up gaps:

A TOCTOU race on the web surfaces (Defect 1). The read-validate-check-save sequence against the two_factor row is not atomic. Two parallel submissions of the same passcode each load their own in-memory copy where LastUsedPasscode still holds the prior value; both pass the inequality check, both authenticate, and both then write the same new value back. Net effect: the same OTP redeems for two independent logged-in sessions.
No LastUsedPasscode check at all on the Basic-Auth API surface (Defect 2). services/auth/basic.go calls twofa.ValidateTOTP(...) for X-Gitea-OTP without ever reading or writing LastUsedPasscode. The same six-digit code is replayable for the full totp.Validate acceptance window (~60–90 s with the default Skew=1). This is a clean RFC 6238 §5.2 violation independent of timing, shaped identically to the pre-CVE-2021-45331 behaviour but scoped to the API / Git-over-HTTPS basic-auth path instead of the web form.

Both defects post-date the 2018 fix; neither is referenced in any published Gitea advisory I could find. I'm filing this as a follow-up to CVE-2021-45331, not a duplicate.

Vulnerable code

Defect 1 — TOCTOU race on web 2FA login

routers/web/auth/2fa.go:55-88:

54  id := idSess.(int64)
55  twofa, err := auth.GetTwoFactorByUID(ctx, id)             // (A) read row
56  ...
62  ok, err := twofa.ValidateTOTP(form.Passcode)              // (B) pure-function RFC 6238 check
...
68  if ok && twofa.LastUsedPasscode != form.Passcode {        // (C) check against in-memory copy
...
84      twofa.LastUsedPasscode = form.Passcode                 // (D) mutate in-memory
85      if err = auth.UpdateTwoFactor(ctx, twofa); err != nil {// (E) UPDATE … AllCols where id=?

Step (E) is plain db.GetEngine(ctx).ID(t.ID).AllCols().Update(t) (models/auth/twofactor.go:128-131) — no row lock, no WHERE last_used_passcode = <previous> predicate, and no DB uniqueness on (uid, last_used_passcode). The model definition at models/auth/twofactor.go:48-57 shows LastUsedPasscode string is a plain column — no constraint, no version field.

Defect 1 — same shape, password-reset 2FA re-auth

routers/web/auth/password.go:179-196 shows the identical pattern in the password-reset flow:

179  passcode := ctx.FormString("passcode")
180  ok, err := twofa.ValidateTOTP(passcode)
...
185  if !ok || twofa.LastUsedPasscode == passcode {           // same check-against-in-memory pattern
...
192  twofa.LastUsedPasscode = passcode
193  if err = auth.UpdateTwoFactor(ctx, twofa); err != nil {

Same shape, same race window.

Defect 2 — Basic-Auth API / Git-over-HTTPS (stateless replay — no check at all)

services/auth/basic.go:170-185:

func validateTOTP(req *http.Request, u *user_model.User) error {
    twofa, err := auth_model.GetTwoFactorByUID(req.Context(), u.ID)
    ...
    if ok, err := twofa.ValidateTOTP(req.Header.Get("X-Gitea-OTP")); err != nil {  // :179
        return err
    } else if !ok {
        return util.NewInvalidArgumentErrorf("invalid provided OTP")
    }
    return nil
}

LastUsedPasscode is neither read nor written on this path. The same six-digit code in X-Gitea-OTP succeeds for the full totp.Validate acceptance window on every request.

Why the existing failed-login counter doesn't catch either defect

Gitea's loginAttempts counter increments on failed sign-ins. A successful replay is a success — the counter is never touched, and two parallel successes produce two access tokens with no anomaly logged at the auth layer.

Race-window analysis (Defect 1)

Inside TwoFactorPost, the critical region between (A) GetTwoFactorByUID and (E) UpdateTwoFactor covers:

one DB SELECT round-trip,
base64-decode + AES-decrypt of the secret (models/auth/twofactor.go:108-118),
totp.Validate (HMAC-SHA1 over the secret + time-step),
user_model.GetUserByID (a second SELECT),
optional linkAccountFromContext / OpenID link branch,
the assignment + UPDATE.

On a non-CPU-bound deployment this window is a few milliseconds on the fast path, tens of ms when linkAccount / OpenID branches are taken. An attacker who already holds both factors and can submit two POST /user/two_factor requests in parallel (HTTP/2 multiplexing, or two backgrounded curls) hits the race reliably — both goroutines enter step (C) with the same stale LastUsedPasscode, both reach step (D), both write the new value back. The two responses each set the user's session and KeyUserHasTwoFactorAuth = true.

The same window exists on the password-reset flow (routers/web/auth/password.go:179-193).

Defect 2 (basic-auth) is a different shape: no race needed. Every request that supplies the correct passcode within totp.Validate's skew window succeeds, indefinitely, until the time-step rolls.

Reachable HTTP routes

Surface	Route	Defect
Web 2FA login	`POST /user/two_factor` (`TwoFactorPost`)	1 — TOCTOU race
Password-reset 2FA re-auth	`POST /user/password/reset` (`ResetPasswdPost`) when `twofa` is set	1 — TOCTOU race
Basic-Auth API	every API endpoint that accepts Basic auth with `X-Gitea-OTP` header (e.g. `/api/v1/user`, `/api/v1/users/{username}/tokens`)	2 — stateless replay
Git-over-HTTPS push/pull	Basic-auth flow, same `X-Gitea-OTP` route into `services/auth/basic.go:validateTOTP`	2 — stateless replay

Proof of concept

Pre-conditions: attacker has the victim's password (credential dump, phish, separate vuln) and one live TOTP value within the RFC 6238 window (AiTM relay such as Evilginx2, malicious browser extension, infostealer log, shoulder-surf). Network reach to the Gitea HTTP listener.

Defect 1 — Web 2FA race (parallel curl)

# Step 1 — start a 2FA-pending session (password phase).
curl -c jar.txt -b jar.txt -d 'user_name=alice&password=<known>' \
     https://gitea.example.com/user/login

# Step 2 — fire two identical POSTs to /user/two_factor with the captured passcode.
PASS=654321
( curl -sS -c jar1.txt -b jar.txt -X POST \
       -d "passcode=${PASS}" https://gitea.example.com/user/two_factor & )
( curl -sS -c jar2.txt -b jar.txt -X POST \
       -d "passcode=${PASS}" https://gitea.example.com/user/two_factor & )
wait

# Step 3 — both cookie jars now hold authenticated sessions for Alice.
curl -b jar1.txt https://gitea.example.com/user/settings  # 200
curl -b jar2.txt https://gitea.example.com/user/settings  # 200

Repeated trials succeed often enough to be exploitable; a kit firing N=5 parallel attempts hits the race on virtually every iteration. Note that the legitimate browser tab counts as one of the racers — the attacker's request only needs to arrive between the victim's (A) and the victim's (E).

Defect 2 — Basic-Auth API replay (no race needed)

# Attacker captured Alice's password + one live OTP (654321).
# Within the RFC 6238 window (~60–90 s):
curl -u "alice:<known-password>" \
     -H "X-Gitea-OTP: 654321" \
     https://gitea.example.com/api/v1/user
# → 200 OK. Repeat as many times as the time-step allows.

Each call succeeds. An attacker can mint a personal access token via POST /api/v1/users/{username}/tokens inside that window for long-lived access that outlives the captured OTP.

Impact

Defect 1 (Web TOCTOU). Narrow exploit window but completely deterministic on parallel submission. The victim's own legitimate login is itself the trigger — no second observation of the OTP is needed if the attacker can race the victim's submission. Net effect: two authenticated sessions for one OTP, defeating RFC 6238 §5.2 in the multi-session case.
Defect 2 (Basic-Auth stateless replay). The more serious of the two. Any captured OTP value remains valid on the API / git-clone basic-auth surface for the full totp.Validate window. An attacker who AiTM-relays one login can carve out 60–90 s of unattended API access during which they can mint a personal access token and persist past the OTP window. This surface specifically attracts attackers because (a) it is non-interactive (a script can hammer it), and (b) PAT minting via /api/v1/users/{username}/tokens does not require a second 2FA prompt once basic-auth + OTP have succeeded.
Successful-replay invisibility. Gitea's failed-login counter increments on FailedLoginException; a successful replay never throws. The audit log records two successful 2FA authentications for the same principal at near-identical timestamps — most SIEM rules will not flag this.

Conditions for exploit

Required	Detail
Network reach to Gitea HTTP listener	Trivially available
Valid victim password	Credential dump / phishing relay / separate vuln
One captured OTP value within ~90 s	AiTM, infostealer log, shoulder-surf, MITM, malicious extension
Ability to fire two parallel HTTP requests	Trivial (`curl -P 2`, `xargs -P 2`, HTTP/2 multiplexing) — Defect 1 only

No special role / permission required on Gitea. Both defects are exploitable from any unauthenticated network position that can reach the listener.

Suggested remediation

Two distinct fixes are needed; option (c) collapses both into one place and is the recommended path.

(a) Race fix — compare-and-swap on UPDATE (Defect 1):

// models/auth/twofactor.go
func UpdateTwoFactorCAS(ctx context.Context, t *TwoFactor, prevPasscode string) (bool, error) {
    n, err := db.GetEngine(ctx).ID(t.ID).
        Where("last_used_passcode = ?", prevPasscode).
        AllCols().Update(t)
    return n == 1, err
}

Each handler captures prev := twofa.LastUsedPasscode before mutating, calls UpdateTwoFactorCAS(ctx, twofa, prev), and rejects the request if n != 1. Fixes both web sites with no extra lock contention. A row-level lock (SELECT … FOR UPDATE inside a db.WithTx) is an equivalent surgical option. Equivalent atomicity can also be obtained by a unique index on (twofa_id, last_used_passcode) so a duplicate UPDATE collides at the DB layer.

(b) Basic-Auth fix (Defect 2):

Wrap the twofa.ValidateTOTP(...) call at services/auth/basic.go:179 in the same inequality check + update pattern used in routers/web/auth/2fa.go:68,84-85, ideally via the CAS helper above so the basic-auth path can't reintroduce the race either.

(c) Preferred — store the accepted time-step counter, route every call site through one consume helper:

Replace LastUsedPasscode string with LastTotpStep int64. Derive the matching step inside TwoFactor.ValidateTOTP (skew-aware) and CAS on the step value:

func (t *TwoFactor) ValidateAndConsumeTOTP(ctx context.Context, passcode string) (bool, error) {
    step, ok, err := validateAndReturnStep(passcode, t.Secret) // skew-aware
    if err != nil || !ok { return false, err }

    n, err := db.GetEngine(ctx).Table("two_factor").
        Where("id = ? AND last_totp_step < ?", t.ID, step).
        Cols("last_totp_step").
        Update(map[string]any{"last_totp_step": step})
    if err != nil || n == 0 { return false, err }   // already consumed — replay refused
    t.LastTotpStep = step
    return true, nil
}

All three call sites (routers/web/auth/2fa.go, routers/web/auth/password.go, services/auth/basic.go) then go through this single function and cannot accidentally skip the consume step. Same fix shape as django-otp (last_t) and Authentik (authentik/stages/authenticator_totp/models.py:184). Recommended option because it makes the defect impossible to reintroduce at a future call site.

A schema migration is required for (c); (a)+(b) is the surgical minimum.

References

RFC 6238 §5.2 — TOTP one-time use: https://datatracker.ietf.org/doc/html/rfc6238#section-5.2
CWE-294 — Authentication Bypass by Capture-replay: https://cwe.mitre.org/data/definitions/294.html
CWE-367 — Time-of-check Time-of-use (TOCTOU) Race Condition: https://cwe.mitre.org/data/definitions/367.html
Original Gitea fix this report builds on — CVE-2021-45331 / PR #3878 (introduced LastUsedPasscode): https://github.com/go-gitea/gitea/pull/3878

Severity

7.1 (High)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:N

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{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "code.gitea.io/gitea"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.5.0"
            },
            {
              "fixed": "1.26.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-20779"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-294"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-21T21:57:24Z",
    "nvd_published_at": "2026-07-03T21:16:56Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\nI\u0027m reporting **two related TOTP one-time-use defects** in Gitea that survive the CVE-2021-45331 fix. The 2018 fix (PR #3878) introduced the `TwoFactor.LastUsedPasscode` field and added an in-memory inequality check on the web 2FA login path. That check works correctly in the single-request case, but it leaves two follow-up gaps:\n\n1. **A TOCTOU race on the web surfaces (Defect 1).** The read-validate-check-save sequence against the `two_factor` row is not atomic. Two parallel submissions of the same passcode each load their own in-memory copy where `LastUsedPasscode` still holds the prior value; both pass the inequality check, both authenticate, and both then write the same new value back. Net effect: the same OTP redeems for two independent logged-in sessions.\n\n2. **No `LastUsedPasscode` check at all on the Basic-Auth API surface (Defect 2).** `services/auth/basic.go` calls `twofa.ValidateTOTP(...)` for `X-Gitea-OTP` without ever reading or writing `LastUsedPasscode`. The same six-digit code is replayable for the full `totp.Validate` acceptance window (~60\u201390 s with the default `Skew=1`). This is a clean [RFC 6238 \u00a75.2](https://datatracker.ietf.org/doc/html/rfc6238#section-5.2) violation independent of timing, shaped identically to the pre-CVE-2021-45331 behaviour but scoped to the API / Git-over-HTTPS basic-auth path instead of the web form.\n\nBoth defects post-date the 2018 fix; neither is referenced in any published Gitea advisory I could find. I\u0027m filing this as a **follow-up** to CVE-2021-45331, not a duplicate.\n\n### Vulnerable code\n\n#### Defect 1 \u2014 TOCTOU race on web 2FA login\n\n`routers/web/auth/2fa.go:55-88`:\n\n```go\n54  id := idSess.(int64)\n55  twofa, err := auth.GetTwoFactorByUID(ctx, id)             // (A) read row\n56  ...\n62  ok, err := twofa.ValidateTOTP(form.Passcode)              // (B) pure-function RFC 6238 check\n...\n68  if ok \u0026\u0026 twofa.LastUsedPasscode != form.Passcode {        // (C) check against in-memory copy\n...\n84      twofa.LastUsedPasscode = form.Passcode                 // (D) mutate in-memory\n85      if err = auth.UpdateTwoFactor(ctx, twofa); err != nil {// (E) UPDATE \u2026 AllCols where id=?\n```\n\nStep (E) is plain `db.GetEngine(ctx).ID(t.ID).AllCols().Update(t)` (`models/auth/twofactor.go:128-131`) \u2014 no row lock, no `WHERE last_used_passcode = \u003cprevious\u003e` predicate, and no DB uniqueness on `(uid, last_used_passcode)`. The model definition at `models/auth/twofactor.go:48-57` shows `LastUsedPasscode string` is a plain column \u2014 no constraint, no version field.\n\n#### Defect 1 \u2014 same shape, password-reset 2FA re-auth\n\n`routers/web/auth/password.go:179-196` shows the identical pattern in the password-reset flow:\n\n```go\n179  passcode := ctx.FormString(\"passcode\")\n180  ok, err := twofa.ValidateTOTP(passcode)\n...\n185  if !ok || twofa.LastUsedPasscode == passcode {           // same check-against-in-memory pattern\n...\n192  twofa.LastUsedPasscode = passcode\n193  if err = auth.UpdateTwoFactor(ctx, twofa); err != nil {\n```\n\nSame shape, same race window.\n\n#### Defect 2 \u2014 Basic-Auth API / Git-over-HTTPS (stateless replay \u2014 no check at all)\n\n`services/auth/basic.go:170-185`:\n\n```go\nfunc validateTOTP(req *http.Request, u *user_model.User) error {\n    twofa, err := auth_model.GetTwoFactorByUID(req.Context(), u.ID)\n    ...\n    if ok, err := twofa.ValidateTOTP(req.Header.Get(\"X-Gitea-OTP\")); err != nil {  // :179\n        return err\n    } else if !ok {\n        return util.NewInvalidArgumentErrorf(\"invalid provided OTP\")\n    }\n    return nil\n}\n```\n\n`LastUsedPasscode` is neither read nor written on this path. The same six-digit code in `X-Gitea-OTP` succeeds for the full `totp.Validate` acceptance window on every request.\n\n#### Why the existing failed-login counter doesn\u0027t catch either defect\n\nGitea\u0027s `loginAttempts` counter increments on **failed** sign-ins. A successful replay is a success \u2014 the counter is never touched, and two parallel successes produce two access tokens with no anomaly logged at the auth layer.\n\n### Race-window analysis (Defect 1)\n\nInside `TwoFactorPost`, the critical region between (A) `GetTwoFactorByUID` and (E) `UpdateTwoFactor` covers:\n\n1. one DB SELECT round-trip,\n2. base64-decode + AES-decrypt of the secret (`models/auth/twofactor.go:108-118`),\n3. `totp.Validate` (HMAC-SHA1 over the secret + time-step),\n4. `user_model.GetUserByID` (a second SELECT),\n5. optional `linkAccountFromContext` / OpenID link branch,\n6. the assignment + UPDATE.\n\nOn a non-CPU-bound deployment this window is a few milliseconds on the fast path, tens of ms when `linkAccount` / OpenID branches are taken. An attacker who already holds both factors and can submit two `POST /user/two_factor` requests in parallel (HTTP/2 multiplexing, or two backgrounded curls) hits the race reliably \u2014 both goroutines enter step (C) with the same stale `LastUsedPasscode`, both reach step (D), both write the new value back. The two responses each set the user\u0027s session and `KeyUserHasTwoFactorAuth = true`.\n\nThe same window exists on the password-reset flow (`routers/web/auth/password.go:179-193`).\n\nDefect 2 (basic-auth) is a different shape: no race needed. Every request that supplies the correct passcode within `totp.Validate`\u0027s skew window succeeds, indefinitely, until the time-step rolls.\n\n### Reachable HTTP routes\n\n| Surface | Route | Defect |\n|---------|-------|--------|\n| Web 2FA login | `POST /user/two_factor` (`TwoFactorPost`) | 1 \u2014 TOCTOU race |\n| Password-reset 2FA re-auth | `POST /user/password/reset` (`ResetPasswdPost`) when `twofa` is set | 1 \u2014 TOCTOU race |\n| Basic-Auth API | every API endpoint that accepts Basic auth with `X-Gitea-OTP` header (e.g. `/api/v1/user`, `/api/v1/users/{username}/tokens`) | 2 \u2014 stateless replay |\n| Git-over-HTTPS push/pull | Basic-auth flow, same `X-Gitea-OTP` route into `services/auth/basic.go:validateTOTP` | 2 \u2014 stateless replay |\n\n### Proof of concept\n\nPre-conditions: attacker has the victim\u0027s password (credential dump, phish, separate vuln) and one live TOTP value within the RFC 6238 window (AiTM relay such as Evilginx2, malicious browser extension, infostealer log, shoulder-surf). Network reach to the Gitea HTTP listener.\n\n#### Defect 1 \u2014 Web 2FA race (parallel curl)\n\n```bash\n# Step 1 \u2014 start a 2FA-pending session (password phase).\ncurl -c jar.txt -b jar.txt -d \u0027user_name=alice\u0026password=\u003cknown\u003e\u0027 \\\n     https://gitea.example.com/user/login\n\n# Step 2 \u2014 fire two identical POSTs to /user/two_factor with the captured passcode.\nPASS=654321\n( curl -sS -c jar1.txt -b jar.txt -X POST \\\n       -d \"passcode=${PASS}\" https://gitea.example.com/user/two_factor \u0026 )\n( curl -sS -c jar2.txt -b jar.txt -X POST \\\n       -d \"passcode=${PASS}\" https://gitea.example.com/user/two_factor \u0026 )\nwait\n\n# Step 3 \u2014 both cookie jars now hold authenticated sessions for Alice.\ncurl -b jar1.txt https://gitea.example.com/user/settings  # 200\ncurl -b jar2.txt https://gitea.example.com/user/settings  # 200\n```\n\nRepeated trials succeed often enough to be exploitable; a kit firing N=5 parallel attempts hits the race on virtually every iteration. Note that the legitimate browser tab counts as one of the racers \u2014 the attacker\u0027s request only needs to arrive between the victim\u0027s (A) and the victim\u0027s (E).\n\n#### Defect 2 \u2014 Basic-Auth API replay (no race needed)\n\n```bash\n# Attacker captured Alice\u0027s password + one live OTP (654321).\n# Within the RFC 6238 window (~60\u201390 s):\ncurl -u \"alice:\u003cknown-password\u003e\" \\\n     -H \"X-Gitea-OTP: 654321\" \\\n     https://gitea.example.com/api/v1/user\n# \u2192 200 OK. Repeat as many times as the time-step allows.\n```\n\nEach call succeeds. An attacker can mint a personal access token via `POST /api/v1/users/{username}/tokens` inside that window for long-lived access that outlives the captured OTP.\n\n### Impact\n\n- **Defect 1 (Web TOCTOU).** Narrow exploit window but completely deterministic on parallel submission. The victim\u0027s own legitimate login is itself the trigger \u2014 no second observation of the OTP is needed if the attacker can race the victim\u0027s submission. Net effect: two authenticated sessions for one OTP, defeating RFC 6238 \u00a75.2 in the multi-session case.\n- **Defect 2 (Basic-Auth stateless replay).** The more serious of the two. Any captured OTP value remains valid on the API / git-clone basic-auth surface for the full `totp.Validate` window. An attacker who AiTM-relays one login can carve out 60\u201390 s of unattended API access during which they can mint a personal access token and persist past the OTP window. This surface specifically attracts attackers because (a) it is non-interactive (a script can hammer it), and (b) PAT minting via `/api/v1/users/{username}/tokens` does not require a second 2FA prompt once basic-auth + OTP have succeeded.\n- **Successful-replay invisibility.** Gitea\u0027s failed-login counter increments on `FailedLoginException`; a successful replay never throws. The audit log records two successful 2FA authentications for the same principal at near-identical timestamps \u2014 most SIEM rules will not flag this.\n\n### Conditions for exploit\n\n| Required | Detail |\n|----------|--------|\n| Network reach to Gitea HTTP listener | Trivially available |\n| Valid victim password | Credential dump / phishing relay / separate vuln |\n| One captured OTP value within ~90 s | AiTM, infostealer log, shoulder-surf, MITM, malicious extension |\n| Ability to fire two parallel HTTP requests | Trivial (`curl -P 2`, `xargs -P 2`, HTTP/2 multiplexing) \u2014 Defect 1 only |\n\nNo special role / permission required on Gitea. Both defects are exploitable from any unauthenticated network position that can reach the listener.\n\n### Suggested remediation\n\nTwo distinct fixes are needed; option (c) collapses both into one place and is the recommended path.\n\n**(a) Race fix \u2014 compare-and-swap on UPDATE (Defect 1):**\n\n```go\n// models/auth/twofactor.go\nfunc UpdateTwoFactorCAS(ctx context.Context, t *TwoFactor, prevPasscode string) (bool, error) {\n    n, err := db.GetEngine(ctx).ID(t.ID).\n        Where(\"last_used_passcode = ?\", prevPasscode).\n        AllCols().Update(t)\n    return n == 1, err\n}\n```\n\nEach handler captures `prev := twofa.LastUsedPasscode` before mutating, calls `UpdateTwoFactorCAS(ctx, twofa, prev)`, and rejects the request if `n != 1`. Fixes both web sites with no extra lock contention. A row-level lock (`SELECT \u2026 FOR UPDATE` inside a `db.WithTx`) is an equivalent surgical option. Equivalent atomicity can also be obtained by a unique index on `(twofa_id, last_used_passcode)` so a duplicate UPDATE collides at the DB layer.\n\n**(b) Basic-Auth fix (Defect 2):**\n\nWrap the `twofa.ValidateTOTP(...)` call at `services/auth/basic.go:179` in the same inequality check + update pattern used in `routers/web/auth/2fa.go:68,84-85`, ideally via the CAS helper above so the basic-auth path can\u0027t reintroduce the race either.\n\n**(c) Preferred \u2014 store the accepted time-step counter, route every call site through one consume helper:**\n\nReplace `LastUsedPasscode string` with `LastTotpStep int64`. Derive the matching step inside `TwoFactor.ValidateTOTP` (skew-aware) and CAS on the step value:\n\n```go\nfunc (t *TwoFactor) ValidateAndConsumeTOTP(ctx context.Context, passcode string) (bool, error) {\n    step, ok, err := validateAndReturnStep(passcode, t.Secret) // skew-aware\n    if err != nil || !ok { return false, err }\n\n    n, err := db.GetEngine(ctx).Table(\"two_factor\").\n        Where(\"id = ? AND last_totp_step \u003c ?\", t.ID, step).\n        Cols(\"last_totp_step\").\n        Update(map[string]any{\"last_totp_step\": step})\n    if err != nil || n == 0 { return false, err }   // already consumed \u2014 replay refused\n    t.LastTotpStep = step\n    return true, nil\n}\n```\n\nAll three call sites (`routers/web/auth/2fa.go`, `routers/web/auth/password.go`, `services/auth/basic.go`) then go through this single function and cannot accidentally skip the consume step. Same fix shape as `django-otp` (`last_t`) and Authentik (`authentik/stages/authenticator_totp/models.py:184`). Recommended option because it makes the defect impossible to reintroduce at a future call site.\n\nA schema migration is required for (c); (a)+(b) is the surgical minimum.\n\n### References\n\n- RFC 6238 \u00a75.2 \u2014 TOTP one-time use: https://datatracker.ietf.org/doc/html/rfc6238#section-5.2\n- CWE-294 \u2014 Authentication Bypass by Capture-replay: https://cwe.mitre.org/data/definitions/294.html\n- CWE-367 \u2014 Time-of-check Time-of-use (TOCTOU) Race Condition: https://cwe.mitre.org/data/definitions/367.html\n- Original Gitea fix this report builds on \u2014 CVE-2021-45331 / PR #3878 (introduced `LastUsedPasscode`): https://github.com/go-gitea/gitea/pull/3878",
  "id": "GHSA-gx3v-q759-g323",
  "modified": "2026-07-21T21:57:24Z",
  "published": "2026-07-21T21:57:24Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/go-gitea/gitea/security/advisories/GHSA-gx3v-q759-g323"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-20779"
    },
    {
      "type": "WEB",
      "url": "https://github.com/go-gitea/gitea/pull/38151"
    },
    {
      "type": "WEB",
      "url": "https://github.com/go-gitea/gitea/commit/99f8b3d9a1d32f4c39828e07971455a18191e0b9"
    },
    {
      "type": "WEB",
      "url": "https://blog.gitea.com/release-of-1.26.3-and-1.26.4"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/go-gitea/gitea"
    },
    {
      "type": "WEB",
      "url": "https://github.com/go-gitea/gitea/releases/tag/v1.26.3"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Gitea: TOTP TOCTOU race on web 2FA paths + missing replay check on Basic-Auth `X-Gitea-OTP` surface"
}

CVE-2026-20779 (GCVE-0-2026-20779)

Vulnerability from cvelistv5 – Published: 2026-07-03 20:19 – Updated: 2026-07-07 17:00

Title

Gitea TOTP single-use enforcement defect allows OTP replay

Summary

Gitea versions from 1.5.0 before 1.26.3 have a TOTP single-use enforcement defect that allows a valid TOTP code to be accepted more than once across web two-factor authentication flows and the Basic Auth X-Gitea-OTP path.

Severity

7.1 (High)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:N

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-294

Assigner

Gitea

References

4 references

URL	Tags
https://github.com/go-gitea/gitea/security/adviso…	vendor-advisory
https://github.com/go-gitea/gitea/pull/38151	patch
https://github.com/go-gitea/gitea/releases/tag/v1.26.3	release-notes
https://blog.gitea.com/release-of-1.26.3-and-1.26.4/	release-notes

Impacted products

1 product

Vendor	Product	Version
Gitea	Gitea Open Source Git Server	Affected: 1.5.0 , < 1.26.3 (semver)

Credits

Kript0r3x

Show details on NVD website

JSON

To clipboard

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  "containers": {
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                "id": "CVE-2026-20779",
                "options": [
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                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-07-06T19:02:05.918693Z",
                "version": "2.0.3"
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          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
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          {
            "tags": [
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            "url": "https://github.com/go-gitea/gitea/security/advisories/GHSA-gx3v-q759-g323"
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        ],
        "title": "CISA ADP Vulnrichment"
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    "cna": {
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          "defaultStatus": "unaffected",
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      "credits": [
        {
          "lang": "en",
          "type": "reporter",
          "value": "Kript0r3x"
        }
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      "descriptions": [
        {
          "lang": "en",
          "value": "Gitea versions from 1.5.0 before 1.26.3 have a TOTP single-use enforcement defect that allows a valid TOTP code to be accepted more than once across web two-factor authentication flows and the Basic Auth X-Gitea-OTP path."
        }
      ],
      "metrics": [
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          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
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            "baseSeverity": "HIGH",
            "confidentialityImpact": "HIGH",
            "integrityImpact": "LOW",
            "privilegesRequired": "NONE",
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            "userInteraction": "REQUIRED",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:N",
            "version": "3.1"
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              "type": "CWE"
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      "providerMetadata": {
        "dateUpdated": "2026-07-03T20:19:29.239Z",
        "orgId": "88ee5874-cf24-4952-aea0-31affedb7ff2",
        "shortName": "Gitea"
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      "references": [
        {
          "name": "GitHub Security Advisory",
          "tags": [
            "vendor-advisory"
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          "url": "https://github.com/go-gitea/gitea/security/advisories/GHSA-gx3v-q759-g323"
        },
        {
          "name": "GitHub Pull Request #38151",
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          ],
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        },
        {
          "name": "Gitea v1.26.4 Release Blog Post",
          "tags": [
            "release-notes"
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    "dateReserved": "2026-03-03T03:26:00.104Z",
    "dateUpdated": "2026-07-07T17:00:54.312Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.

Action not permitted

GHSA-GX3V-Q759-G323

Summary

Vulnerable code

Defect 1 — TOCTOU race on web 2FA login

Defect 1 — same shape, password-reset 2FA re-auth

Defect 2 — Basic-Auth API / Git-over-HTTPS (stateless replay — no check at all)

Why the existing failed-login counter doesn't catch either defect

Race-window analysis (Defect 1)

Reachable HTTP routes

Proof of concept

Defect 1 — Web 2FA race (parallel curl)

Defect 2 — Basic-Auth API replay (no race needed)

Impact

Conditions for exploit

Suggested remediation

References

CVE-2026-20779 (GCVE-0-2026-20779)

Tags

Sightings

Nomenclature