Bug 2565678 (CVE-2026-121996)

Summary: CVE-2026-121996 gdb: gdbserver: heap disclosure and wild pointer in gdb_parse_agent_expr via untrusted agent-expression length
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: high Docs Contact:
Priority: high    
Version: unspecifiedCC: rhel-process-autobot, security-response-team, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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Doc Text:
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.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description OSIDB Bzimport 2026-11-03 10:03:12 UTC 
Length validation flaw in gdb_parse_agent_expr (gdbserver/ax.cc). The function trusts the attacker-declared xlen unconditionally: xmalloc(xlen) allocates the buffer and aexpr->length is set to xlen, but hex2bin stops at the packet terminator if actual data is shorter, leaving the buffer tail uninitialized. Additionally, *actparm is advanced by xlen*2 regardless of actual data length, producing a wild pointer the caller dereferences. Two confirmed impacts: (1) gdbserver echoes uninitialized heap bytes back to the client, disclosing heap addresses for ASLR bypass; (2) deterministic crash via wild pointer when declared length greatly exceeds actual data.