CVE-2025-68973: Critical Memory Corruption Flaw in GnuPG Armor Parsing Engine

Aegiron 12 mins0

CVE ID: CVE-2025-68973
Affected Software: GnuPG
Affected Versions: All GnuPG releases up to and including 2.4.8
CVSS v3.x Score: 7.8 (High)
Severity: High
Vulnerability Type: Out-of-Bounds Write (CWE-787)
Attack Vector: Local
Attack Complexity: Low
Privileges Required: None
User Interaction: Required
Exploitability: Moderate to High
Exploit Availability: Proof-of-concept techniques publicly discussed; no fully weaponized exploit published, but exploitation is considered feasible for skilled attackers
Patched Version: GnuPG 2.4.9 and later

Vulnerability Overview

CVE-2025-68973 is a memory corruption vulnerability in GnuPG’s handling of ASCII-armored data. The flaw sits in the armor parsing logic, a core feature used whenever GnuPG processes PGP keys, encrypted messages, or signed content in the familiar -----BEGIN PGP----- text format.

At a high level, the problem is simple but dangerous: a variable that tracks where data should be written in memory is incremented twice instead of once. That single logic mistake is enough to push a write operation past the end of its intended buffer. When GnuPG processes specially crafted armored input, it can end up writing data into memory it does not own.

Because GnuPG is widely used for encryption, signature verification, package signing, and secure email, this bug has security implications far beyond a simple crash.

Technical Description

The vulnerability resides in the armor_filter function in the g10/armor.c source file. This function is responsible for encoding and decoding ASCII-armored PGP data, including handling headers, base64 payloads, and checksums.

During decoding, the function maintains an index variable that tracks the current position within a buffer. Due to faulty control flow, this index is incremented twice in one execution path when only a single increment was intended. As a result:

  • The index advances farther than expected.
  • Subsequent write operations occur outside the allocated buffer.
  • No sufficient boundary check prevents this out-of-range write.

When processing normal input, this logic error may never surface. However, when the input is deliberately crafted with specific lengths, line breaks, and base64 patterns, the parser is forced into the vulnerable path. At that point, the double increment causes memory corruption.

This is a classic out-of-bounds write condition, which can lead to crashes, data corruption, or — under the right circumstances — attacker-controlled code execution.

Exploitation Methodology

Exploitation relies on delivering maliciously crafted ASCII-armored data to a GnuPG process. The general flow looks like this:

  1. Crafting the input
    The attacker builds a PGP armored block with carefully chosen structure:
    • Valid-looking armor headers and footers
    • Base64 sections with precise lengths and padding
    • Content designed to steer the parser into the faulty increment path
  2. Triggering parsing
    The victim system processes the malicious input. This can happen when:
    • Importing a public key
    • Decrypting an encrypted message
    • Verifying a signed file or message
  3. Memory corruption
    When armor_filter processes the crafted data, the index variable is incremented twice. This causes writes to occur beyond the allocated buffer, corrupting adjacent memory.
  4. Impact escalation
    Depending on memory layout and execution context, the attacker may achieve:
    • Denial of Service: GnuPG crashes due to a segmentation fault
    • Memory Corruption: Internal structures are overwritten, leading to unpredictable behavior
    • Code Execution: In advanced scenarios, overwritten control data (such as function pointers or heap metadata) could redirect execution
    • Privilege Escalation: If GnuPG runs with elevated privileges or processes data for privileged workflows, attacker code may execute with higher permissions

While reliable code execution is non-trivial, the vulnerability is serious enough that skilled attackers are assumed capable of developing functional exploits.

Realistic Attack Scenarios

Email-Based Attacks
A victim receives an encrypted or signed email containing malicious armored data. When they decrypt or verify the message using GnuPG, the vulnerability is triggered.

Key Server Poisoning
An attacker uploads a specially crafted public key to a key server. Users who import that key unknowingly process the malicious armor and expose their system.

Software Supply Chain Abuse
In build systems or CI pipelines that rely on GnuPG to verify signatures, a malicious signature or key could compromise the build environment, leading to downstream tampering.

Local Privilege Escalation
On shared systems, a low-privileged user places malicious armored files in locations processed by automated jobs or privileged users. When those jobs run, exploitation may lead to elevated access.

MITRE ATT&CK Mapping

  • Initial Access (TA0001)
    • Phishing: Spearphishing Attachment (T1566.001)
  • Execution (TA0002)
    • Exploitation for Client Execution (T1203)
  • Persistence (TA0003)
    • Compromise Client Software Binary (T1554)
  • Privilege Escalation (TA0004)
    • Exploitation for Privilege Escalation (T1068)
  • Defense Evasion (TA0005)
    • Obfuscated Files or Information (T1027)

Proof of Concept Status

Security researchers have demonstrated proof-of-concept techniques that reliably trigger the out-of-bounds write. These PoCs show crashes and detectable memory corruption when GnuPG processes crafted armored input.

Although no fully weaponized exploit achieving public, turnkey code execution has been released, the underlying bug is well understood. Given the maturity of exploitation techniques for memory corruption issues, organizations should assume that capable threat actors can build working exploits.

Detection Strategies

Behavioral Indicators

  • Unexpected GnuPG crashes, especially segmentation faults during key import, decryption, or verification
  • Memory access violations logged by the operating system
  • Unusual processing behavior, such as excessive memory usage or long hangs when handling armored files

Log Sources to Monitor

  • System Logs:
    • Linux: syslog / journald entries for segfaults or core dumps
    • Windows: Application Error events involving gpg.exe
  • Application Logs:
    • GnuPG debug output showing malformed armor parsing or decoding errors
  • Email Gateway Logs:
    • Encrypted messages or attachments with abnormal armored content
  • Endpoint Detection & Response (EDR):
    • Abnormal memory access patterns
    • Unexpected child processes spawned by GnuPG
  • Audit Logs:
    • Suspicious file access patterns where GnuPG processes untrusted armored files

Detection Signatures and Rules

Network/File Inspection:

alert tcp any any -> any any (
  msg:"Possible CVE-2025-68973 exploitation - malformed GnuPG armor";
  content:"-----BEGIN PGP";
  pcre:"/-----BEGIN PGP[^-]{1,50}?-----/i";
  sid:1000001;
)

YARA Rule for Suspicious Armored Files:

rule CVE_2025_68973_Suspicious_Armor {
    strings:
        $armor_begin = "-----BEGIN PGP"
        $armor_end   = "-----END PGP"
        $odd_pattern = { 3D [4-8] 0D 0A }
    condition:
        $armor_begin and $armor_end and
        (#odd_pattern > 15) and
        filesize < 100KB
}

Splunk Crash Detection Query:

index=system sourcetype=syslog (gpg OR gnupg)
("segmentation fault" OR "core dumped" OR "signal 11")
| stats count by host, user
| where count > 2

Windows Event Log Indicators:

  • Event ID 1000 or 1001
  • Application name: gpg.exe or gpg2.exe
  • Exception code: 0xc0000005 (Access Violation)

Remediation and Mitigation

Official Patch

The GnuPG development team has fixed this issue in GnuPG version 2.4.9. The patch corrects the faulty index logic in armor_filter and adds defensive checks to prevent similar issues.

Action: Upgrade all GnuPG installations to 2.4.9 or later using official distribution packages or verified source builds.

Immediate Mitigation Steps

  • Upgrade Immediately: This is the most effective and recommended fix.
  • Pre-Scan Inputs: Validate armored files for obvious malformations before passing them to GnuPG.
  • Sandboxing: Run GnuPG in containers or restricted environments where possible.
  • Least Privilege: Avoid running GnuPG with administrative or root privileges unless absolutely required.
  • Enhanced Monitoring: Enable alerts for crashes, memory faults, and abnormal GnuPG behavior.

Verification After Patching

  • Confirm gpg --version reports 2.4.9 or higher
  • Test key import, encryption, decryption, and signature verification
  • Review logs to ensure no residual parsing errors or crashes remain

Impact Assessment

  • Confidentiality: High
    • Successful exploitation may expose private keys or decrypted data
  • Integrity: High
    • Memory corruption can undermine trust in cryptographic verification
  • Availability: Medium
    • Crashes and denial-of-service conditions are likely during exploitation

Organizations that rely on GnuPG for secure communications, software signing, or automated verification workflows should treat CVE-2025-68973 as a priority issue and remediate without delay.

Aegiron

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