BREW-DSTACK-CVE-2026-49265 (GHSA-XPV3-W29H-X7CV)
Vulnerability from osv_homebrew – Published: 2026-09-30 19:10 – Updated: 2026-11-09 08:53 – Source websiteSummary
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
- Attacker intercepts
authorization_codevia Custom URI Scheme Hijacking - PKCE blocks token request — attacker lacks
code_verifier - Attacker sends repeated requests to
/tokenendpoint measuring response times - Using timing oracle, attacker recovers
code_verifiercharacter by character - 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
{
"affected": [
{
"ecosystem_specific": {
"fix": "bump",
"range_state": "fixed",
"resource": "oauthlib",
"resource_purl": "pkg:pypi/oauthlib@4.0.1",
"upstream_fixed_in": "4.0.0"
},
"package": {
"ecosystem": "Homebrew",
"name": "dstack",
"purl": "pkg:brew/dstack"
},
"ranges": [
{
"events": [
{
"introduced": "0.9.1"
},
{
"fixed": "0.22.1_1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"database_specific": {
"confidence": "high",
"source": "matched",
"strategy": "registry",
"upstream_evidence": [
{
"ecosystem": "PyPI",
"key": "pkg:pypi/oauthlib@4.0.1",
"name": "oauthlib",
"resource": "oauthlib",
"strategy": "registry",
"subject_version": "4.0.1"
}
]
},
"details": "## Summary\n\nA timing side-channel vulnerability exists in the PKCE (RFC 7636) implementation \nof the Authorization Code Grant flow. The `code_challenge_method_plain` function \nuses Python\u0027s standard `==` operator for string comparison instead of a \nconstant-time comparison function, potentially allowing timing-based attacks.\n\n## Affected Component\n\n- File: `oauthlib/oauth2/rfc6749/grant_types/authorization_code.py`\n- Functions: `code_challenge_method_plain`, `code_challenge_method_s256`\n- Vulnerability Type: CWE-208 (Observable Timing Discrepancy)\n\n## Technical Details\n\nPython\u0027s `==` operator uses short-circuit evaluation when comparing strings:\n1. Returns `False` immediately if lengths differ\n2. Compares characters left-to-right, stopping at first mismatch\n\nThis means comparison time varies linearly with the length of the common prefix \nbetween the attacker-supplied verifier and the stored challenge, creating a \nmeasurable timing oracle.\n\n## Proof of Concept\n\nTested locally against oauthlib source (network jitter eliminated to isolate \npure Python execution time):\n\n| Input | Result | Time (10M iterations) |\n|---|---|---|\n| Wrong first char (`B` + `A`*49) | Fast reject | 0.34106s |\n| 49 chars correct (`A`*49 + `B`) | Deep compare | 0.37847s |\n| **Difference** | | **0.03741s** |\n\nThe ~37ms delta over 10M iterations corresponds to nanosecond-level differences \nper call, which are statistically exploitable under controlled conditions.\n\n## Attack Scenario\n\n1. Attacker intercepts `authorization_code` via Custom URI Scheme Hijacking\n2. PKCE blocks token request \u2014 attacker lacks `code_verifier`\n3. Attacker sends repeated requests to `/token` endpoint measuring response times\n4. Using timing oracle, attacker recovers `code_verifier` character by character\n5. Attacker obtains Access Token \u2192 Account Takeover\n\n\u003e **Note:** Practical exploitability is limited due to the single-use nature of \n\u003e authorization codes and real-world network noise. However, the vulnerable \n\u003e pattern should be corrected as a defense-in-depth measure.\n\n## Recommended Fix\n\nReplace `==` with `hmac.compare_digest()` for constant-time comparison:\n\ncr: Elvin Latifli",
"id": "BREW-dstack-CVE-2026-49265",
"modified": "2026-11-09T08:53:19Z",
"published": "2026-09-30T19:10:36Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/oauthlib/oauthlib/security/advisories/GHSA-xpv3-w29h-x7cv"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-49265"
},
{
"type": "WEB",
"url": "https://github.com/oauthlib/oauthlib/pull/963"
},
{
"type": "WEB",
"url": "https://github.com/oauthlib/oauthlib/commit/40b0ab56da3682c2484a4b78bbff309f8025d950"
},
{
"type": "PACKAGE",
"url": "https://github.com/oauthlib/oauthlib"
},
{
"type": "WEB",
"url": "https://github.com/pypa/advisory-database/tree/main/vulns/oauthlib/PYSEC-2026-4114.yaml"
}
],
"schema_version": "1.7.3",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N",
"type": "CVSS_V3"
}
],
"summary": "Oauthlib: Timing Attack Vulnerability in PKCE code_verifier Comparison (CWE-208)",
"upstream": [
"GHSA-xpv3-w29h-x7cv",
"CVE-2026-49265",
"PYSEC-2026-4114"
]
}
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