CVE-2026-50631 Overview
CVE-2026-50631 is a race condition vulnerability in Apache CXF affecting the AbstractOAuthDataProvider class. Concurrent requests using the same refresh token can bypass single-use semantics when recycleRefreshTokens is set to false. The flaw allows attackers to replay a leaked refresh token across multiple threads and obtain several valid access tokens from a single token redemption. The Apache CXF project has released fixed versions 4.2.2 and 4.1.7 to address the issue. The vulnerability is tracked under [CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition].
Critical Impact
A leaked refresh token can be replayed concurrently to mint multiple valid OAuth access tokens, undermining token revocation and session boundaries.
Affected Products
- Apache CXF versions prior to 4.2.2 (4.2.x branch)
- Apache CXF versions prior to 4.1.7 (4.1.x branch)
- Deployments using AbstractOAuthDataProvider with recycleRefreshTokens=false
Discovery Timeline
- 2026-06-12 - CVE-2026-50631 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2026-50631
Vulnerability Analysis
Apache CXF implements OAuth 2.0 token management through the AbstractOAuthDataProvider class. The refresh token flow is intended to enforce single-use semantics, where each refresh token redemption invalidates the original token and issues a new access token. When the recycleRefreshTokens configuration is set to false, the provider keeps the original refresh token usable across multiple redemptions.
The defect lies in the absence of proper synchronization around the token validation and access token generation steps. Two or more concurrent requests carrying the same refresh token pass the validity check before any update occurs, and each request proceeds to mint a separate access token. The result is a violation of the OAuth specification's expectation that refresh token redemption is atomic.
An attacker who obtains a refresh token through interception, log exposure, or credential theft can issue parallel requests to multiply valid access tokens. This expands the blast radius of a single leak and complicates revocation efforts because multiple access tokens remain valid until natural expiry.
Root Cause
The root cause is a non-atomic check-then-act sequence in the refresh token redemption path. The provider validates the refresh token state and generates a new access token without holding a lock or using a compare-and-swap operation against the token store. This [CWE-367] TOCTOU race window allows concurrent threads to observe the same valid state.
Attack Vector
The vulnerability is exploitable over the network without authentication beyond possession of the refresh token. An attacker submits multiple simultaneous grant_type=refresh_token requests against the OAuth token endpoint. Each request returns a distinct valid access token. The attack requires precise timing because the race window is narrow, which is reflected in the high attack complexity.
No public proof-of-concept exploit is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. See the Apache Mailing List Thread for upstream details.
Detection Methods for CVE-2026-50631
Indicators of Compromise
- Multiple successful token endpoint responses returning different access tokens with identical or near-identical timestamps for the same refresh_token value.
- Bursts of OAuth refresh requests from a single client identifier within milliseconds.
- Authorization server logs showing concurrent refresh token grants tied to the same subject and client.
Detection Strategies
- Correlate access token issuance events by refresh token identifier and alert when more than one issuance occurs within a short time window.
- Inspect HTTP access logs on OAuth token endpoints for parallel POST requests carrying the same refresh token payload.
- Apply behavioral analytics to detect anomalous spikes in access token volume per refresh token, especially where recycleRefreshTokens=false is configured.
Monitoring Recommendations
- Enable verbose audit logging on the Apache CXF OAuth data provider and forward events to a centralized SIEM.
- Track access token-to-refresh token ratios per client and baseline normal behavior to surface deviations.
- Monitor for downstream API requests using multiple access tokens sharing a common refresh token lineage.
How to Mitigate CVE-2026-50631
Immediate Actions Required
- Upgrade Apache CXF to version 4.2.2 for the 4.2.x branch or version 4.1.7 for the 4.1.x branch.
- Inventory all services using AbstractOAuthDataProvider and identify deployments where recycleRefreshTokens is set to false.
- Revoke active refresh tokens after upgrading to invalidate any tokens that may have been replayed prior to patching.
Patch Information
The Apache CXF project has released versions 4.2.2 and 4.1.7 containing the fix. The patched releases add synchronization around the refresh token redemption path to enforce atomic check-and-update semantics. Refer to the Apache Mailing List Thread and the OpenWall OSS Security Discussion for vendor advisory details.
Workarounds
- Set recycleRefreshTokens to true so that each redemption invalidates the previous refresh token and reduces the race window impact.
- Shorten refresh token lifetimes to minimize the value of any leaked token until upgrade is complete.
- Place a rate-limiting reverse proxy in front of the OAuth token endpoint to reject burst requests carrying the same refresh token.
# Configuration example: enforce refresh token recycling in Apache CXF
# Set on AbstractOAuthDataProvider bean configuration
<property name="recycleRefreshTokens" value="true"/>
<property name="accessTokenLifetime" value="3600"/>
<property name="refreshTokenLifetime" value="86400"/>
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