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Fix PDF decryption issue for some empty password files
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Some PDF's with an empty password can't be decrypted. Investigation
found that the problem is a strlen check to prevent an overflow rather
than passing down the actual length of the allocated field.

Specifically, the UE buffer may have NULL values in it, so a strlen
check will claim the field is shorter than it is and then later checks
fail because the length is the wrong size.

While at it, I improved code comments on the function reading dictionary
key-value strings and switched a flag use a bool rather than an int.
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@micahsnyder
micahsnyder committed Nov 3, 2023
1 parent 47c079f commit 814325a
Showing 1 changed file with 23 additions and 13 deletions.
36 changes: 23 additions & 13 deletions libclamav/pdf.c
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Original file line number Diff line number Diff line change
Expand Up @@ -123,7 +123,7 @@ static void XFA_cb(struct pdf_struct *pdf, struct pdf_obj *obj, struct pdfname_a
static int pdf_readint(const char *q0, int len, const char *key);
static const char *pdf_getdict(const char *q0, int *len, const char *key);
static char *pdf_readval(const char *q, int len, const char *key);
static char *pdf_readstring(const char *q0, int len, const char *key, unsigned *slen, const char **qend, int noescape);
static char *pdf_readstring(const char *q0, int len, const char *key, unsigned *slen, const char **qend, bool noescape);

static int xrefCheck(const char *xref, const char *eof)
{
Expand Down Expand Up @@ -2120,7 +2120,7 @@ static void pdf_parse_trailer(struct pdf_struct *pdf, const char *s, long length

pdf->flags |= 1 << ENCRYPTED_PDF;
pdf_parse_encrypt(pdf, enc, s + length - enc);
newID = pdf_readstring(s, length, "/ID", &newIDlen, NULL, 0);
newID = pdf_readstring(s, length, "/ID", &newIDlen, NULL, false);

if (newID) {
free(pdf->fileID);
Expand Down Expand Up @@ -2571,7 +2571,18 @@ static const char *pdf_getdict(const char *q0, int *len, const char *key)
return q;
}

static char *pdf_readstring(const char *q0, int len, const char *key, unsigned *slen, const char **qend, int noescape)
/**
* @brief Read the value string from a PDF dictionary key/value pair.
*
* @param q0 A pointer into the PDF dictionary.
* @param len The bytes remaining in the file.
* @param key The key we're looking for.
* @param [out] slen The length of the output string
* @param [out] qend The pointer we wound up at, after the end of the value.
* @param noescape Select 'true' to ignore escape characters, 'false' to process them.
* @return char*
*/
static char *pdf_readstring(const char *q0, int len, const char *key, unsigned *slen, const char **qend, bool noescape)
{
char *s, *s0;
const char *start, *q, *end;
Expand Down Expand Up @@ -2870,7 +2881,7 @@ static void compute_hash_r6(const char *password, size_t pwlen, const unsigned c

static void check_user_password(struct pdf_struct *pdf, int R, const char *O,
const char *U, int32_t P, int EM,
const char *UE,
const char *UE, size_t UE_len,
unsigned length, unsigned oulen)
{
unsigned i;
Expand All @@ -2891,11 +2902,9 @@ static void check_user_password(struct pdf_struct *pdf, int R, const char *O,
cl_sha256(U + 32, 8, result2, NULL);
dbg_printhex("Computed U", (const char *)result2, 32);
if (!memcmp(result2, U, 32)) {
size_t UE_len;

/* Algorithm 3.2a could be used to recover encryption key */
cl_sha256(U + 40, 8, result2, NULL);
UE_len = UE ? strlen(UE) : 0;

if (UE_len != 32) {
cli_dbgmsg("check_user_password: UE length is not 32: %zu\n", UE_len);
noisy_warnmsg("check_user_password: UE length is not 32: %zu\n", UE_len);
Expand Down Expand Up @@ -2927,10 +2936,9 @@ static void check_user_password(struct pdf_struct *pdf, int R, const char *O,

compute_hash_r6(password, pwlen, (const unsigned char *)(U + 32), validationkey);
if (!memcmp(U, validationkey, sizeof(validationkey))) {
size_t UE_len;

compute_hash_r6(password, pwlen, (const unsigned char *)(U + 40), hash);
UE_len = strlen(UE);

if (UE_len != 32) {
cli_dbgmsg("check_user_password: UE length is not 32: %zu\n", UE_len);
noisy_warnmsg("check_user_password: UE length is not 32: %zu\n", UE_len);
Expand Down Expand Up @@ -3103,6 +3111,7 @@ void pdf_handle_enc(struct pdf_struct *pdf)
struct pdf_obj *obj;
uint32_t len, n, R, P, length, EM = 1, i, oulen;
char *O, *U, *UE, *StmF, *StrF, *EFF;
size_t UE_len = 0;
const char *q, *q2;

if (pdf->enc_objid == ~0u)
Expand Down Expand Up @@ -3213,7 +3222,8 @@ void pdf_handle_enc(struct pdf_struct *pdf)
length = 128;
} else {
n = 0;
UE = pdf_readstring(q, len, "/UE", &n, NULL, 0);
UE = pdf_readstring(q, len, "/UE", &n, NULL, false);
UE_len = n;
length = 256;
}
}
Expand All @@ -3222,7 +3232,7 @@ void pdf_handle_enc(struct pdf_struct *pdf)
length = 40;

n = 0;
O = pdf_readstring(q, len, "/O", &n, NULL, 0);
O = pdf_readstring(q, len, "/O", &n, NULL, false);
if (!O || n < oulen) {
cli_dbgmsg("pdf_handle_enc: invalid O: %d\n", n);
cli_dbgmsg("pdf_handle_enc: invalid O: %d\n", n);
Expand All @@ -3244,7 +3254,7 @@ void pdf_handle_enc(struct pdf_struct *pdf)
}

n = 0;
U = pdf_readstring(q, len, "/U", &n, NULL, 0);
U = pdf_readstring(q, len, "/U", &n, NULL, false);
if (!U || n < oulen) {
cli_dbgmsg("pdf_handle_enc: invalid U: %u\n", n);
noisy_warnmsg("pdf_handle_enc: invalid U: %u\n", n);
Expand All @@ -3271,7 +3281,7 @@ void pdf_handle_enc(struct pdf_struct *pdf)
noisy_warnmsg("pdf_handle_enc: wrong key length, not multiple of 8\n");
break;
}
check_user_password(pdf, R, O, U, P, EM, UE, length, oulen);
check_user_password(pdf, R, O, U, P, EM, UE, UE_len, length, oulen);
} while (0);

free(O);
Expand Down

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