VulnerabilityPublished 6 Aug 2026

CVE-2026-71326: Traefik · BasicAuth singleflight key collision allows authenticated identity spoofing

Affects github.com/traefik/traefik/v3Source GCVE Database

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Not known to be exploited · CVSS 2.1 Low : upgrade to 3.6.25 when convenient.

Data as of 5 Nov 2026, 00:01 UTC

CVSS 4.0 base score

2.1Low

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.3% chance of exploitation in 30 days – higher than 27% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.6.25, 3.7.10

CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: Present
Privileges Required: High
User Interaction: None
Vulnerable system confidentiality: Low
Vulnerable system integrity: Low
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: Present: The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: a race condition must be won to successfully exploit the vulnerability (the successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker, or the attack may need to be launched multiple times against a single target before being successful); the attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
Privileges Required: High: The attacker requires privileges that provide significant (e.g., administrative) control over the vulnerable system allowing full access to the vulnerable system’s settings and files.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the Vulnerable System.
Vulnerable system integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact to the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 6 Aug 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 5 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade github.com/traefik/traefik/v3 to 3.6.25; github.com/traefik/traefik/v3 to 3.7.10.

Affected software

Product	Affected	Fixed in
Gogithub.com/traefik/traefik/v3	Affected >=3.6.11 <3.6.25	Fixed in 3.6.25
Gogithub.com/traefik/traefik/v3	Affected >=3.7.0 <3.7.10	Fixed in 3.7.10

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A patch is available and should be applied. Upgrade Traefik to version 3.6.25 or later in the 3.6 line, or version 3.7.10 or later in the 3.7 line to resolve this vulnerability. These versions include a fix that encodes the password length in the singleflight key to prevent collisions. No additional mitigation is required if the patch is applied.

Description

## Summary There is a low severity vulnerability in Traefik's BasicAuth middleware. Concurrent password verifications are deduplicated through a singleflight group whose key was the delimiter-free concatenation of the submitted password and the stored secret, so a request carrying an unconfigured username — whose secret is empty — can produce the same key as a configured user's valid request and receive that request's successful result. Exploitation requires the attacker to already hold a valid credential **and** to read the stored password hash, which is only reachable through paths that are themselves privileged: the API is documented as admin-only, the Kubernetes path requires read access to the Secret, and the Docker path requires access to the socket. The key now encodes the password length as a prefix, so distinct (password, secret) pairs can no longer collide. Only the v3.6 line from v3.6.11 onwards and the v3.7 line are affected; earlier v3 releases and the v2 line do not carry the vulnerable deduplication path. ## Patches - https://github.com/traefik/traefik/releases/tag/v3.6.25 - https://github.com/traefik/traefik/releases/tag/v3.7.10 ## For more information If you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues). <details> <summary>Original Description</summary> ## Summary Traefik's BasicAuth middleware deduplicates concurrent password checks with a `singleflight.Group`. Its key is the delimiter-free concatenation `password + secret`. For an existing user with password `P` and stored hash `H`, the key is `P || H`. An unknown user can select the password `P || H`; because its secret is the empty string, its key is also `P || H`. If the existing user's request starts the shared calculation, the unknown user receives the existing user's successful Boolean result. Traefik then continues processing the unknown user's original request and propagates the attacker-selected username through `URL.User`, the access log, and the configured BasicAuth `headerField`. A user who knows one valid username/password/hash tuple can therefore authenticate concurrently under any unconfigured username. This becomes a privilege escalation when a backend uses the BasicAuth `headerField` as a trusted identity, which is the documented purpose of that option. ## Details The vulnerable logic is in `pkg/middlewares/auth/basic_auth.go:118-131`: ```go func (b *basicAuth) checkPassword(user, password string) bool { secret := b.auth.Secrets(user, b.auth.Realm) key := password + secret match, _, _ := b.singleflightGroup.Do(key, func() (any, error) { if secret == "" { _ = b.checkSecret(password, b.notFoundSecret) return false, nil } return b.checkSecret(password, secret), nil }) return match.(bool) } ``` For a configured user `viewer`: ```text password = P secret = H key = P || H result = true ``` For an unconfigured user `admin`: ```text password = P || H secret = "" key = (P || H) || "" = P || H ``` `singleflight.Group.Do` shares the first in-flight result for equal keys. If the configured user's check is first, the unknown user's closure is not run and the unknown request receives `true`. The authorization result is not bound to the username. After the shared result is accepted, `ServeHTTP` uses the username parsed from the unknown request: ```go req.URL.User = url.User(user) if b.headerField != "" { req.Header.Del(b.headerField) req.Header[b.headerField] = []string{user} } ``` Consequently, the backend sees the attacker-selected `admin` identity, not the valid request's `viewer` identity. ### Attack prerequisites The attacker needs: 1. network access to a route protected by the affected BasicAuth middleware; 2. one valid low-privilege username and password; 3. the corresponding stored password hash. The hash is often present in deployment labels or routing configuration. Traefik's API is also a direct source when the attacker can access it: `GET /api/http/middlewares/{id}` serializes `basicAuth.users`, including the hash, despite the field carrying `loggable:"false"`. The official v3.7.8 binary returned the hash in the validation environment. The attacker does not need another user's password or a victim-generated request. The attacker creates both concurrent requests: one with their valid credentials and one with an arbitrary, unconfigured target username. ### Security impact When `headerField` is configured, an authenticated low-privilege user can impersonate an arbitrary identity to the backend. Depending on downstream authorization, this can allow: - access to administrative data; - execution of privileged state-changing operations; - corruption of audit attribution; - bypass of identity-based tenant or role separation. Without `headerField`, the unknown request is still admitted through the BasicAuth middleware. The practical consequence then depends on whether the protected route treats

Impact

An attacker with a valid low-privilege username and password and access to the corresponding stored password hash can impersonate any unconfigured username to the backend when the BasicAuth middleware's headerField option is used. This can lead to privilege escalation, unauthorized access to administrative data, execution of privileged operations, corruption of audit logs, and bypass of identity-based access controls. The attack requires privileged access to retrieve the stored password hash, which is typically protected by admin-only API access or restricted Kubernetes/Docker permissions.

Summary

Traefik's BasicAuth middleware uses a singleflight.Group to deduplicate concurrent password verification requests by concatenating the submitted password and stored secret as a key. Because the key was formed without delimiters, an attacker who holds a valid credential and knows the corresponding stored password hash can craft a request with an unconfigured username and a password equal to the concatenation of the valid password and hash. This causes the singleflight mechanism to share the authentication result, allowing the attacker to bypass authentication checks and impersonate arbitrary usernames. The vulnerability affects Traefik v3.6 from 3.6.11 up to 3.6.25 (exclusive) and v3.7 from 3.7.0 up to 3.7.10 (exclusive). The issue is fixed by encoding the password length as a prefix in the key, preventing collisions.

CWE-287Improper Authentication (opens in a new tab)
Class weaknessLikelihood of exploit: HighOWASP Top 10 A07:2025: Authentication Failures (opens in a new tab)
When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Architecture and Design · Libraries or Frameworks
Use an authentication framework or library such as the OWASP ESAPI Authentication feature.
OWASP Cheat Sheets
OWASP Authentication (opens in a new tab)
OWASP ASVS 4.0.3
V2.8.5 (opens in a new tab)
V2.8.4 (opens in a new tab)
V9.2.3 (opens in a new tab)
Attack patterns
Fake the Source of Data (opens in a new tab)
Utilizing REST's Trust in the System Resource to Obtain Sensitive Data (opens in a new tab)
Session Hijacking (opens in a new tab)
Token Impersonation (opens in a new tab)
Upload a Web Shell to a Web Server (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 6 Aug 2026, 18:19 UTC

WeaknessesCWE-287

Technical details

References

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