CVE-2025-24898

Executive Summary

CVE-2025-24898 is a use-after-free vulnerability in the rust-openssl crate (versions prior to 0.10.70), specifically in the ssl::select_next_proto function used for Application-Layer Protocol Negotiation (ALPN) callback handling. Applications that construct the server argument buffer within the ALPN callback are vulnerable to a memory safety issue where the returned slice carries a lifetime bound to the client argument instead of the server argument, potentially leading to heap memory disclosure or server crash. The vulnerability is remotely exploitable in standard network-facing TLS implementations where the ALPN callback triggers use-after-free conditions during TLS handshake processing.

Technical Analysis

The vulnerability resides in the ssl::select_next_proto function within the rust-openssl crate's SSL module. The function is responsible for comparing the client's ALPN protocol list against the server's supported protocols and returning a selected protocol slice. The core issue is lifetime mismanagement:

Root Cause Mechanics:

In affected versions, the function signature improperly binds the output lifetime to the client parameter only:

pub fn select_next_proto<'a>(client: &'a [u8], server: &'a [u8]) -> Option<&'a [u8]>

While the function internally returns a reference to the server buffer, the compiler is misled into believing the returned reference is valid for the lifetime of the client argument. In typical callback usage:

ctx.set_alpn_select_callback(|_, client_protocols| {
    let server_protocols = b"\x02h2\x08http/1.1";  // Stack-allocated, short lifetime
    ssl::select_next_proto(client_protocols, server_protocols)
});

The server_protocols buffer is stack-allocated within the callback and destroyed when the closure returns. If the caller (OpenSSL C library or downstream code) retains the returned pointer beyond the callback invocation — which occurs when the ALPN selection result is used after the callback completes — the pointer references freed stack memory.

Memory Safety Implications:

1. Use-After-Free: Accessing the returned slice after the callback scope ends reads from freed memory, which may have been reused by other operations.
2. Information Disclosure: The freed stack memory may contain transient data (credentials, session keys, buffers from concurrent threads) that gets transmitted to the client during TLS handshake protocol advertisement.
3. Server Crash: If the freed memory has been unmapped or overwritten with an invalid pointer structure, dereferencing can cause a segmentation fault.

Affected Execution Flow:

1. Client initiates TLS handshake with ALPN extension listing protocols
2. OpenSSL invokes the Rust ALPN callback
3. Callback constructs server buffer (via static data, file read, or stack allocation)
4. select_next_proto returns a slice pointing to freed memory
5. OpenSSL reads from this dangling pointer to negotiate the protocol
6. Resulting memory read exposes stale data or crashes the worker process

The vulnerability specifically manifests when the server buffer is constructed within the callback scope. If the server protocol list is a static/global constant with 'static lifetime, the bug does not manifest. This narrows the practical exploit surface but does not eliminate it — dynamic protocol negotiation, plugin-driven protocol lists, or templates loaded per-request are all affected patterns.

Attack Surface:

• Public-facing TLS servers (HTTPS, WSS, gRPC)
• Servers using rustls-compatible OpenSSL bindings for ALPN
• Applications implementing HTTP/2 or HTTP/3 protocol negotiation
• Multi-tenant services where ALPN list varies by virtual host configuration

Exploitation Scenarios

Scenario: Remote Heap Memory Leak via ALPN Negotiation

1. Reconnaissance/Target Identification:

   • Attacker scans for services advertising h2 or http/1.1 via TLS ALPN on common ports (443, 8443, 9443)
   • Fingerprint rust-openssl by sending crafted ALPN extensions and observing response inconsistencies
   • Target servers running services compiled against rust-openssl < 0.10.70

2. Trigger/Weaponization:

   • Craft a TLS ClientHello with ALPN extension listing multiple protocols: \x02h2\x08http/1.1\x09spdy/3.1
   • Include an entry that forces the server to traverse a dynamic protocol comparison path
   • Arrange the server buffer allocation timing: send the ClientHello immediately after server performs a sensitive operation (e.g., private key decryption, session ticket creation) so freed stack memory contains interesting data

3. Payload/Execution:

   • Connect via raw TLS socket using OpenSSL or custom TLS client
   • Send ClientHello with ALPN extension containing protocol list that matches the server's offered protocol at a specific byte offset
   • Repeat handshakes across multiple connections to sample different freed stack regions
   • Parse the ALPN ServerHello response — extract the protocol name bytes returned by the server
   • Exfiltrate adjacent memory contents that may have been copied into the ALPN response

4. Post-Exploitation Impact:

   • Reconstruct stack memory layout across multiple handshakes to harvest:
     • Session ticket keys
     • HTTP/2 header compression context (HPACK dynamic table entries)
     • Authentication tokens from concurrent requests
     • Secrets from neighboring worker threads
   • Combine leaked memory with other vulnerabilities for credential theft or TLS session hijacking
   • Use the crash variant to mount a denial-of-service attack by sending handshakes that cause the freed stack region to be unmapped, terminating worker processes

Scenario: TLS DoS via ALPN Crash

1. Attacker floods the server with rapid TLS handshakes containing malformed ALPN extensions
2. Each handshake triggers the callback where server buffer is constructed and destroyed
3. Sending thousands of parallel connections causes the freed stack to be reused aggressively
4. Eventually a reference to unmapped memory (e.g., after guard page overlap) causes SIGSEGV, crashing the worker and terminating legitimate connections

Impact Assessment

Severity Assessment: While no CVSS score is officially assigned, this vulnerability exhibits characteristics consistent with a High severity rating (7.0–8.9) based on:

• Confidentiality Impact: High — Arbitrary stack memory contents (including TLS session keys, request data, authentication tokens) may be disclosed to remote unauthenticated attackers
• Integrity Impact: Low — The attacker does not directly modify server data without additional primitives
• Availability Impact: High — Exploitation can deterministically crash server processes (SIGSEGV)
• Attack Vector: Network — Exploitable via standard TLS handshake on any exposed ALPN endpoint
• Complexity: Low to Medium — Requires understanding of ALPN negotiation but no special privileges

Blast Radius:

• Rust ecosystem: All Rust services using OpenSSL bindings for TLS termination, including web servers (via hyper/actix-warp wrappers), reverse proxies, gRPC services, and database connection pools
• The openssl crate is a transitive dependency in thousands of crates — any project using native-tls or direct openssl bindings pulls the vulnerable code
• Cloud services: Managed Rust workloads on AWS Lambda, GCP Cloud Run, and Azure Functions that terminate TLS at the application layer
• Containerized deployments: Any Rust-based microservice with TLS configured to self-terminate connections

Exploitation Difficulty Assessment:

• No public exploit code exists
• Requires a server that constructs ALPN lists dynamically within callbacks — less common but present in configurable frameworks
• Memory layout control is probabilistic; repeated handshakes improve success rates
• Information disclosure is more achievable than the crash variant, requiring careful timing of client connections

Detection Guidance

Sigma Rule

title: Detect Malicious ALPN Extension Patterns in TLS ClientHello
id: 4a7c5b2e-8f44-4d3a-9e6b-1c2f3a4b5c6d
status: experimental
description: Detects TLS ClientHello packets with unusually large or malformed ALPN extension lists potentially targeting rust-openssl CVE-2025-24898
logsource:
  category: network_connection
  product: tls
detection:
  selection:
    event.action: (tls_handshake OR tls_client_hello)
  alpn:
    tls.extension.alpn: exists
  condition:
    filter_size:
      tls.extension.alpn.length: > 50
    filter_anomaly:
      tls.extension.alpn: "h2" OR "http/1.1" OR "spdy/3.1"
  condition: selection and (filter_size or filter_anomaly)
  timeframe: 10s
fields:
  - source.ip
  - destination.ip
  - tls.extension.alpn
falsepositives:
  - Legitimate HTTP/2 clients advertising standard protocol lists
  - Misconfigured TLS clients with verbose ALPN extensions
level: medium
tags:
  - attack.t1190
  - cve.2025-24898

YARA Rule

rule Detect_RustOpenSSL_ALPN_Exploit_Attempt {
    meta:
        author = "Valters IT Security Team"
        description = "Detects binary patterns in TLS handshake payloads targeting CVE-2025-24898"
        reference = "CVE-2025-24898"
        date = "2025-02-01"
        severity = "high"
    strings:
        $alpn_ext = { 00 10 00 00 ( [0-50] ) }  // ALPN extension with large list
        $h2_proto = "h2" ascii fullword
        $http11 = "http/1.1" ascii fullword
        $spdy = "spdy/3.1" ascii fullword
        $fuzzed = { FF FF FF FF [0-200] }       // Malformed length fields
    condition:
        uint16(0) == 0x0303 and $alpn_ext and any of them and #h2_proto > 3
}

Suricata IDS Rule

alert tcp any any -> any 443 (msg:"ET EXPLOIT Rust OpenSSL ALPN Use-After-Free Attempt CVE-2025-24898"; flow:established,to_server; content:"|16|"; depth:1; content:"|03|"; within:2; content:"|00 10|"; distance:9; within:6; content:"|00 00|"; distance:2; within:4; threshold:type both, track by_src, count 10, seconds 5; classtype:attempted-dos; sid:20250201; rev:1;)

Note: Rule triggers on high-frequency TLS handshakes with ALPN extension, indicating reconnaissance or crash attempts.

Elastic Detection

sequence by source.ip with maxspan=10s
  [network where event.action == "connection_established" and destination.port == 443]
  [network where event.action == "tls_client_hello" and tls.extension.alpn.size() > 3]
  [network where event.action == "connection_terminated" and network.error_code == "TLS_PROTOCOL_ERROR"]

Detects a pattern of rapid TLS handshake initiation with multiple ALPN protocols followed by protocol errors, indicating exploitation attempts.

Splunk SPL Query

index=network sourcetype=zeek:ssl
| where tls_extension_alpn NOT NULL
| eval alpn_count = mvcount(split(tls_extension_alpn, ","))
| where alpn_count > 3
| bucket _time span=1m
| stats dc(dest_ip) as unique_targets, count as attempt_count by _time, src_ip
| where attempt_count > 50
| eval rule_name = "Potential CVE-2025-24898 ALPN exploitation attempt"
| table _time, src_ip, unique_targets, attempt_count, rule_name

Wazuh OSSEC Rule

<group name="rust_openssl_alpn_cve_2025_24898,">
  <rule id="316530" level="10">
    <if_sid>240001</if_sid>
    <regex>h2|http/1.1|spdy/3.1</regex>
    <regex>ALPN|protocol negotiation|use.after.free|SIGSEGV</regex>
    <description>CVE-2025-24898: Repeated ALPN handshake errors - potential use-after-free exploitation attempt in rust-openssl TLS service</description>
    <group>tls,exploitation,</group>
    <mitre><id>T1190</id></mitre>
    <options>no_full_log</options>
  </rule>
  <rule id="316531" level="7">
    <if_sid>2489810</if_sid>
    <field name="sca_severity">High</field>
    <description>CVE-2025-24898: Confirmed TLS crash after ALPN negotiation - service impact likely</description>
    <group>impact,availability,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 3/4 (LIKELY — 2 independent sources confirm fix).

1) Immediate Mitigations (Do this now):

• Disable ALPN callbacks temporarily: If direct protocol negotiation is not business-critical, set SslContextBuilder::set_alpn_select_callback to a no-op or use the static-selection variant set_alpn_select_callback_str with 'static strings
• Use static buffer allocations: Ensure the server buffer passed to select_next_proto uses 'static lifetime (e.g., const PROTOCOLS: &[u8] = b"\x02h2\x08http/1.1"; declared at module level)
• Restrict network exposure: Apply WAF/network ACL rules to block TLS handshakes with malformed ALPN extensions exceeding 3 protocol entries
• Enable supervisor restarts: Configure process supervisors (systemd, docker restart policies) to auto-restart crashed worker processes to mitigate DoS impact

2) Patch/Upgrade Steps:

• Immediate package update — Upgrade the openssl crate to version 0.10.70 or later:

  # Cargo.toml
  [dependencies]
  openssl = "0.10.70"
  

  cargo update -p openssl
  

• Verify patched version: cargo tree | grep openssl should show openssl v0.10.70+
• Rebuild and redeploy: All Rust services must be recompiled against the new crate; the fix is a compile-time lifetime constraint and requires no runtime migration
• Check transitive dependencies: Use cargo deny or cargo audit to identify all crates depending on vulnerable openssl version and trigger rebuilds
• Test TLS paths: After upgrade run full TLS regression tests including HTTP/2 negotiation, WebSocket upgrade, and custom ALPN protocols

3) Long-Term Hardening:

• Adopt rustls: For new Rust services, evaluate migration from OpenSSL bindings to rustls (pure-Rust TLS library) to reduce C-level dependency bloat
• Implement memory safety audits: Add cargo-audit to CI/CD pipelines to fail builds on known-vulnerable crate versions
• Harden TLS endpoints: Deploy TLS termination at the load balancer or reverse proxy level (nginx, Envoy, HAProxy) to prevent application-level TLS parsing attacks
• Enforce privilege separation: Run TLS termination workers with minimal OS privileges and memory access restrictions (seccomp, AppArmor profiles)
• Continuous monitoring: Maintain real-time monitoring for crash loops in TLS worker processes and abnormal ALPN negotiation counts as early-warning indicators

References

1. NVD Entry — https://nvd.nist.gov/vuln/detail/CVE-2025-24898
2. rust-openssl GitHub Advisory — https://github.com/sfackler/rust-openssl/security/advisories (Oct 2024 advisory referencing lifetime fix)
3. RustSec Advisory DB — https://rustsec.org/advisories/ (tracking advisory for CVE-2025-24898, published as RUSTSEC-2025-XXXX)
4. Crates.io Package Registry — https://crates.io/crates/openssl (version 0.10.70 release notes)
5. CISA Known Exploited Vulnerabilities Catalog — Not listed (no in-the-wild exploitation confirmed as of reporting date)

Threat Actor Activity Assessment: As of this report, no specific threat actor campaigns targeting CVE-2025-24898 have been publicly identified. The vulnerability's exploitation requires specific server-side code patterns, limiting its attractiveness for mass exploitation. However, given the ubiquity of the openssl crate in the Rust ecosystem and the information disclosure potential, expect opportunistic exploitation attempts once public PoC code emerges. Monitoring should focus on anomalous TLS handshake patterns and unexplained worker process crashes in Rust-based services.