VulnerabilityPublished 30 Sep 2026Updated 10 Nov 2026

CVE-2026-49265: Oauthlib · Timing Attack Vulnerability in PKCE code_verifier Comparison

Affects vdirsyncerSource GCVE Database

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Not known to be exploited · CVSS 6.8 Medium : upgrade to 0.21.0_1 when convenient.

Data as of 10 Nov 2026, 23:34 UTC

CVSS 3.1 base score

6.8Medium

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

Patch: Available – fixed in 0.21.0_1

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

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: Required
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: None

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 30 Sep 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 10 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 vdirsyncer to 0.21.0_1.

Affected software

Product	Affected	Fixed in
Homebrewvdirsyncer`pkg:brew/vdirsyncer`Other threats	Affected >=0.16.7 <0.21.0_1	Fixed in 0.21.0_1

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

Description

## Summary A timing side-channel vulnerability exists in the PKCE (RFC 7636) implementation of the Authorization Code Grant flow. The `code_challenge_method_plain` function uses Python's standard `==` operator for string comparison instead of a constant-time comparison function, potentially allowing timing-based attacks. ## Affected Component - File: `oauthlib/oauth2/rfc6749/grant_types/authorization_code.py` - Functions: `code_challenge_method_plain`, `code_challenge_method_s256` - Vulnerability Type: CWE-208 (Observable Timing Discrepancy) ## Technical Details Python's `==` operator uses short-circuit evaluation when comparing strings: 1. Returns `False` immediately if lengths differ 2. Compares characters left-to-right, stopping at first mismatch This means comparison time varies linearly with the length of the common prefix between the attacker-supplied verifier and the stored challenge, creating a measurable timing oracle. ## Proof of Concept Tested locally against oauthlib source (network jitter eliminated to isolate pure Python execution time): | Input | Result | Time (10M iterations) | |---|---|---| | Wrong first char (`B` + `A`*49) | Fast reject | 0.34106s | | 49 chars correct (`A`*49 + `B`) | Deep compare | 0.37847s | | **Difference** | | **0.03741s** | The ~37ms delta over 10M iterations corresponds to nanosecond-level differences per call, which are statistically exploitable under controlled conditions. ## Attack Scenario 1. Attacker intercepts `authorization_code` via Custom URI Scheme Hijacking 2. PKCE blocks token request — attacker lacks `code_verifier` 3. Attacker sends repeated requests to `/token` endpoint measuring response times 4. Using timing oracle, attacker recovers `code_verifier` character by character 5. Attacker obtains Access Token → Account Takeover > **Note:** Practical exploitability is limited due to the single-use nature of > authorization codes and real-world network noise. However, the vulnerable > pattern should be corrected as a defense-in-depth measure. ## Recommended Fix Replace `==` with `hmac.compare_digest()` for constant-time comparison: cr: Elvin Latifli

References

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