CVE-2026-123610: 
Linux Debian vulnerability analysis and mitigation

CVE-2026-123610 is an uncontrolled memory allocation vulnerability in the rustls TLS library affecting all versions before 0.23.46. The flaw resides in rustls's optional Brotli certificate decompression feature, which limits decompressed output size but incorrectly accepts an attacker-controlled large history window. A malicious peer can send a crafted five-byte Brotli header requesting up to 1 GiB of memory allocation before TLS authentication completes, potentially crashing Brotli-enabled clients or client-authenticating servers. Disclosed on November 8, 2026, it carries a CVSS v3.1 base score of 3.7 (Low) and a CVSS v4.0 base score of 6.3 (Medium) (GitHub Advisory, GitHub Advisory DB).

The root cause is CWE-770 (Allocation of Resources Without Limits or Throttling): rustls calls BrotliDecompress with Brotli's large-window extension enabled, applying size limits only to the decompressed output rather than to internal allocations such as the history window buffer. An attacker can craft a minimal five-byte Brotli header that specifies a history window of up to 1 GiB, causing the allocator to attempt satisfying this request before any certificate authentication occurs. Exploitation requires the target to have the brotli feature enabled — clients must connect to a malicious server, while servers must both enable Brotli and require client certificate authentication; neither scenario requires a trusted or valid attacker certificate (GitHub Advisory).

Successful exploitation results in a denial-of-service condition through memory exhaustion: if the system's allocator cannot satisfy the up to 1 GiB allocation request, the affected rustls client or server process will abort or crash. There is no impact on confidentiality or integrity — the vulnerability is limited to availability. Applications most at risk are those that explicitly enable the brotli Cargo feature, including TLS clients connecting to untrusted servers and mTLS servers accepting client certificates from untrusted peers (GitHub Advisory, GitHub Advisory DB).

No public proof-of-concept exploit code or evidence of in-the-wild exploitation has been observed as of the disclosure date (GitHub Advisory DB). The EPSS score is approximately 0.165% (6th percentile), indicating a low near-term exploitation probability. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. Exploitation is constrained by the requirement that the target must have explicitly opted into the non-default brotli feature, raising the effective attack complexity.

1. Reconnaissance: Identify rustls-based TLS clients or servers (e.g., via banner analysis or dependency scanning) that have the brotli Cargo feature enabled and are running versions prior to 0.23.46.
2. Set up a malicious TLS endpoint: For targeting clients, operate a rogue TLS server; for targeting servers with mTLS, act as a malicious client presenting a crafted certificate.
3. Craft malicious Brotli certificate: Construct a TLS certificate compressed with Brotli that contains a five-byte header specifying the maximum allowed history window (up to 1 GiB), while keeping the actual payload minimal.
4. Initiate TLS handshake: Connect to the target (or accept a connection from the target) and present the malicious Brotli-encoded certificate during the TLS handshake, before authentication completes.
5. Trigger memory exhaustion: The vulnerable rustls instance calls BrotliDecompress, which attempts to allocate the attacker-specified history window. If the allocator cannot satisfy the request, the process aborts, achieving denial of service (GitHub Advisory).

• Logs: Sudden process crashes or OOM (out-of-memory) kill events in system logs (dmesg, journalctl) associated with rustls-based services; application logs showing abrupt TLS handshake failures with memory allocation errors.
• Network: Unusual or repeated TLS connection attempts from unknown peers to services using rustls with client certificate authentication enabled; short-lived connections that terminate during the handshake phase.
• Process: Unexpected termination of rustls-based service processes (e.g., via SIGABRT or SIGKILL from the OOM killer) without corresponding application-level errors; elevated memory usage spikes during TLS handshake processing.

Update rustls to version 0.23.46 or later (also fixed in 0.24.0-dev.2), which corrects the Brotli decompression to properly bound internal allocations (GitHub Advisory). If immediate patching is not feasible, disable the brotli Cargo feature in your rustls dependency, as it is disabled by default and only affects applications that explicitly opt in. Additionally, implementing system-level memory limits (e.g., via cgroups or ulimit) on services running affected versions can reduce the impact of a successful exploitation attempt.

The vulnerability was responsibly disclosed by OpenAI's OutboundDisclosures team to the rustls maintainers, who credited them in the security advisory (GitHub Advisory). Community discussion has been limited given the low severity rating and the non-default nature of the affected feature. The CVE was noted on infosec.exchange and tracked by standard vulnerability aggregators shortly after publication.